Sheet aluminum planographic plate



Jan. 25, v1938. w. B. wEscoTT 2,105,358

SHEET ALUMINUM PLANQGRAPHIC PLATE .Filed Nov. 8, 1955 jg/ffl mentalen.25, 193s PATENT OFFICE SHEET ALUMINUMl PLANOGBAPHIC PLATE William B.Wescott, Dover, Mass., assignor, by mesne assignments, toAddressograph-Multigraph Corporation, Cleveland, Ohio, a corporation ofDelaware Application November s, 1935, serial No. 48,935

9 Claims.

This invention relates to a method of preparing sheet aluminum forplanographic printing and to the product of said method; and itcomprises treating sheet aluminum in a hot aqueous solution of an alkalialuminate and an alkali hydroxide whereby all traces of grease areremoved and both major surfaces of said sheet are pitted.

and sometimes, but not always, coated with a layer of microporous oxide,and thereafter rins- 10 ing the said so-treated sheet aluminum andsometimes, but not always, washing the same to remove any superficialscum which mayresult from said treatment, and then drying the product;whereby are obtained sheet aluminum planographic printing plates, bothmajor surfaces of V which are grained with fine pits characterized by acontained deposit of microporous oxide and which surfaces are sometimes,but not always, further characterized by a surface layer of mi- 2ocroporous oxide comprising a trace of alkali, and k which surfaces arebasic in reaction and highly sensitized to direct image-formingmaterials comprising free fatty acids; all as more fully hereinafterdescribed and claimed. The art of planographic printing from metalplates depends, as does the older lithographie art, upon theimmiscibility of oil and water and upon the preferential retention of agreasy imageforming substance by the image areas, and a similarretention of an aqueous dampening uid by the non-image areas. In orderto condition A a metal printing plate for these preferential retentions,it has heretofore been customary to remove all traces of grease and thento counteretch the plate to provide a basic surface wherewith the fattyacid ,component of the greasy image-forming material might' react. A.sotreated metal plate is said to be "sensitized vto ink. Thesensitization" or counter-etching 40 operation has heretofore beenperformed by the user and usually immediately preceding the delineationof the image on the plate. After the greasy image has been delineated inknown manner thereupon, the non-image areas are customarily etched o rdesensitized by treatment with an aqueous solution adapted to at leastneutralize the basic condition previously produced,

and preferably to render these areas lyophilic. In general it has beenthe custom to "desensitize" by theuse of a weak acid and to .create alyophilic condition by means of an adsorbable lyophillc collod, ofwhich'gurn arabic is the most commonly used example. It is also knownthat certain salts of the metal are preferentially water-wettable inthemselves, and hence if present do not require the adsorption of alyophilic col- 1pm. when the image to be printed has been directlydelineated upon the printing surface by means of crayon or the like, itis customary then to desensitize the non-printing areas, thereafter towash out theV image with turpentine, and to replace it with asphaltum orsome other image-forming material which will take a firmer grip on theplate and hence betterwithstand the attritional effect of the inkingrolls during printing; thus assuring of the. integrity of the imagetrirougnoutedinons of many thousands of copies.

Metal plates are vcustomarily roughened or grained forl the purpose ofminimizing the attritional effect of the ink rollers on the image and,

the maintenanceA perhaps more importantly, .for'the purpose of ipreventing these rollers from too greatly reducing the ilm of moisturewhich must bel retained .in order -to prevent ink contamination of thenon-printing image. 'I'his grain g 'operation is customarily performedby gyrat marbles over ilne sand on the plate; and it is a costly, time-`consuming operation requiring much skill.

It isan object of -this invention to obviate the l necessity formechanical graining of planographic plates. It is another object of thisinvention to obviate the heretofore necessary step ofl counter-etching,or sensitization, by the planographic operator. It is likewise an objectof this invention `to obtain a requisite grain and sensitization to inksolely by chemical treatment of the plate. It is a further object ofthis invention to prepare aluminum plates for planographic printing bytreatment with a single chemical reagent whereby a plate is cleaned, l.e., freed from that contamination by grease which is inevitable -in theproduction of sheet aluminum, and is provided simultaneously with anadequate grain and a highly grease-sensitive surface.

Another object of this invention is to provide aluminum planographicprinting plates which are highly sensitized to direct image-formingsubstances comprising fatty acidsand yet are equally serviceable for theretention of and printing from photographically produced images. It l.isanotherobject to produce aluminum planographic printing .plates whereofboth major surfaces are conditioned for the reception and retention ofplanographlc printing images, and in particular for such images whenthey are directly delineated upon' the said surfaces, as by pen, crayon,or typewriter. A still further .object is to produce planographicprinting plates of sheet aluminum of which both sides are conditionedfor use and may be stored indefinitely; and which, after such storageand without further preparation, will firmly retain an image delineatedwith a greasy image-forming material comprising free fatty acid withoutthe necessity for any counter-etching or supplementary.

treatment to sensitize said plates to grease for ima'ge reception andretention. It is yet another object of this invention to provideplanographic printing plates of sheet aluminum characterized by minutesurface pits containing a. vdeposit of microporous oxide of the metaland further characterized by an alkaline reacting surface. It is also anobject to provide a nely pittedor grained aluminum planographic printingplate with a surface layer of relatively soft microporous materialcomprising an oxide of the metal and an alkali, whereby said plates areparticularly sensitive to ordinary carbon paper.

It is an object of this invention to provide a process whereby sheetaluminum may be conditioned to receive and to retain a planographicprinting image, and particularly a directly delineated image, by atreatment with a single chemical reagent which simultaneously removesall traces ofgrease (both superficial grease and that ground into themetal during the rolling process to which the sheet aluminum issubjected in manufacture) and minutely pits or grains the -surfaces anddeposits within the pits an adherent layer of relatively softmicroporous material comprising an oxide of the metal and an alkali. Itis an object of another embodiment of this invention to provide aprocessas above described which, in addition, will coat the said minutelypitted plates throughout their major surfaces with a layer of the saidmicroporous material.

Among the advantages of the process of this invention may be mentionedthe fact that the cost of preparation of the sheet aluminum forplanographic printing is greatly reduced by the elimination of the usualcostly and tedious mechanical graining operation. Anothervadvantage ofthe said process lies in the fact that the aluminum is thereby bothgrained and sensitized, whereby the necessity for "counter-etching orsensitizing by the user is avoided. Still another advantage is that bothsides of the plates of ,this invention are simultaneously prepared foruse and this without resort to building up laminated structures as hasbeen heretofore suggested.

Among the advantagesv of the plates of this invention may be mentionedthe fact that they are useful for the reception and retention of imagesdirectly delineated by greasy image-forming materials comprising fattyacids, and for photographicimages as well. Another advantage of the saidplates is that the high sensitivity to fatty acids imparted by theprocess of this invention is not diminished by long storage, and yettheir grease-sensitive surfaces may be readily desensitized by suchweakly acidic etches as are buered at or about the isoelectric point ofusual photographic image-forming' materials. Still another advantage ofthe plates of this invention is that they are particularly retentive ofpoorly coherent greasy image-forming materialssuch as the more or lesspulverulent coating of ordinary carbon paper. A still further advantageof the said plates is that they *arev adapted to print from an image, asdirectly. delineated in crayon or the. like, without the necessity ofwashing out the original image forming material and the replacementthereof with asphaltum or the like as is common practice, and thus theiruse affords much saving of time and expense. Moreover, and by reason ofthe firm retention of the image as originally directly delineated on theplate, much less skill is required than is requisite when the originalwork has to be washed out and redeveloped, as is the usual practice; andthis is of material advantage. Other objects and advantages will Vbecomeapparent as the description proceeds.

In the acompanying drawing is illustrated more or less diagrammaticallya plate formed in accordance with this invention;

^ Fig. 1 shows a portion of a plate I0 embodying this invention having acharacter ii upon the surface I2 thereof;

Figs. 2 and 3 are enlarged sectional views showing theplate with pits i5 therein and provided with a coating or layer i6, in Fig. 2 the pitsalone being coated and in Fig. 3 both the pits and surface being coated;and y Fig. 4 is an enlarged plan view of the plate.

Relatively pure aluminum is particularly suited to the purposes of thisinvention; and it is preferred to use aluminum in the higher grades ofpurity, although that. alloy of aluminum and manganese designated by itsmanufacturers and known to the trade as 38H may be satisfactorily used,and with particular advantage if a heavy deposit in the pits of theplate or over the pitted surface is desired.

a portion of A bath suitable for the practice of this inven tion may bemade by dissolving sodium aluminate in ordinary tap water. The sodiumaluminate may be replaced either in part or in whole by other alkalialuminates, such as potassium aluminate; but sodium aluminate alone ispreferred. While it has been found convenient to employ the bestcommercial grade of sodium aluminate, so- ,called white soluble NaAlOz,the compounds may however be made as required by well-known methods,`such as by the reaction of sodium hydroxide with metallic aluminum or analuminum salt.

It has been observed that the temperature of the bath, the concentrationof alkali in the bath, the time of treatment, together with the ratio ofaluminum surface to bath volume are to some extent compensatingvariables. Thus,-differences in concentration can be offset andsubstantially the same result obtained by inverse adjustment' ofthetemperature or time of treatment, or both; and also by varying the ratioof surface tobath volume.

In general, a bath is preferred which contains five grams of the 90%sodium aluminate referred to above for every cc. of water. Best resultsare obtained when the bath is maintained at a temperature slightly belowthe boiling point. A temperature of F. plus or minus 5 has been foundsatisfactory. When operating continuously, a ratio of one square foot ofaluminum surface to every five gallons of the preferred solution and animmersion time of 11/2 minutes provide a satisfactory relationship.4 Asubstantially equivalent relationship for processing by the batch methodconsists of a ratio of one square foot of aluminum surface to every twoand one-half gallons of solution, and an immersion time of 11/2 minuteswith a pause between batches of 11/2 minutes. If there were nopausebetween batches,- the relationship wouldbe the same as that given forthe continuous operation, for reasons which will later become apparent.

One of the desirable effects of treating aluminum in a bathof an alkalialuminate is an ero-v sion or pitting of the surface of the metal. Thiserosion is accompanied by loss of Weight and loss of thickness of thesheet aluminum. 'I'he treatment is-considered to be at itsOptimum when-maximum pitting occurs with minimum loss of.'

thickness. The optimum treatment of sheet aluminum according to thisinvention may be because adherence to'these conditions has yielded`satisfactory results in practice, and partly because they areparticularly convenient.

The behavior of hot alkali alumlnate baths, asf

influenced by reaction with commercial alurninum, is not whollyunderstood: but certain theoretical considerations are-hereinafter givenas being useful in clarifying the probable relation- Aship of severalfactors, although not relied upon as factual. y

A freshly prepared solution of sodium aluminate in the preferredconcentration of five grams of the commercial substance to every 100 cc.of water appears to bel substantially stable over a period of lmanyhours. When such a hot solution is reacted with commercial aluminum (asfor instance that grade designated by its makers as 3SH), the bath losesits initial stability and the sodium aluminate hydrolyzes to formaluminum hydroxide or oxide, and sodium hydroxide. The former isprecipitated, for the most part, as a sediment; the latter is deemed tobe the active agent of the bath, and it reacts with the aluminum to formsodium aluminate. In so `doing it erodes or pits the metal andcorrespondingly exhausts itself. For any given initial concentration ofsodium aluminate, the amount of sodium hydroxide in the bath at anytime, after hydrolysis has been initiated by the reactive presencetherein of metallic aluminum and under otherwise constant conditions,depends upon the ratio of the surface of 'aluminum immersed in the bathto the volume of the bath.

Disregarding such small amounts of impurities as there may be, alkali ispresent in the bath as sodium hydroxide and as sodium aluminate, andeach canbe determined as such by titration. It is convenient to expressthe concentration of sodium hydroxide in terms of mol. percent; thatthat is to say that the ratio of the mols of sodium hydroxide to themols of total alkali present may be expressed in percentage. Whilesatisfactory plate surface treatment is obtainable from alkali aluminatebaths over a wide range of molal concentrations of sodium hydroxide, ithas been found that about 15 mol. percent sodium hydroxide concentrationcorresponds to substantial equilibrium at an expedientrate of treatmentand at a convenient ratio of bath volume to immersed aluminum surface.

Freshly made solutions of commercial 90% sotitration to have between 1land`16 mol. percentl sodium hydroxide. When aluminum is first im- 100cc. of water and maintained at about 195 F mersed in such a freshly madesolution; the mol.

, percent of sodium hydroxide drops rapidly as' the aluminum reactstherewith; and then; as hydrolysis is in some unknown way stimulated asa consequence of this reaction, the mol. percent of sodium hydroxiderises again. By adjustment of u one or more ofthe several mentionedvariables,

the mol. percent of sodium hydroxide may be maintained within the rangeof that of the bath as freshly made; and, as stated, a molal concen-ltration of I15% of sodium hydroxide has been found convenient andsatisfactory. Thus, when there is kept immersed about one square foot ofaluminum surface in five gallons of solution con-y taining flve grams ofsodium aluminate for every the bath will remain in substantialequilibrium at about 15 mol. percent sodium hydroxide concentration andthe optimum treatment of the Y,

aluminum surface will obtain in about 11/2 minutes. The loss of weightof aluminum is direct- `ly proportional to the time of immersion; and,

, and some solution is entrained with the metal when it is removed fromthe bath, it is periodically necessary to replace both. It has beenfound convenient to replace lost water by such additions more solutionthan is occasioned by drawing a continuous ribbon of sheet aluminumthrough abath. Continuous operation is preferred.

When operating by the batch method, the individual sheets of aluminumare preferably maintained in a substantially vertical position andspaced apart by at least two inches. In continuous operation the-desired substantially verticall position of the aluminum is sucientlyobtained by feeding thegribbon of metal through the bath in one or moreloops such that for the most part the ribbon is vertically pendanttherein.

Pjor certain purposes, such as forming a direct image by means of agreasy substance comprising very little free fatty acid or a substanceof such friableor pulverulent a nature as topoorly contact theplate'surface, it has been found desirable as part of this invention tosubstanti-ally coat the entire surface of the plate with' suchmicroporous material as is discernibly retained only in the pits of theplates made as above described.

ns a means of coatingthe entire surface of sheet aluminum with the saidmicroporous material and as a means particularly effective whenrelatively thick coatings are desired, i. e., coatsingle bath made up of1.5 of sodium aluminate to every 100 cc. of Water and adjusted to amolal concentration of sodium hydroxide between 5 and 10 percent, withan immersion time of ten minutes. Under these conditions, the loss ofweight is of the order of one "gram per square foot of surface and theloss in thickness is neglithe metal the grease commonly associatedtherewith.

As stated, aluminum hydroxide is precipitated as a sediment; and itis`convenient to allow this sediment to collect in the bottom of thetank and to remove it at daily intervals. Best'results are obtained whenthe sediment is not agitated, although avoidance of agitation is notessential. It is, however, essential for uniform results and thereforedesirable that the metalbeing treated should be kept from contact withthe concentrated ing, the aluminum is swabbedwith a soft rubber -spongeor with cotton wadding. When aluminum alloyed with manganese (that knownto .the

--trade as 3SH) is treated in the alkali baths of this invention, itbecomes loosely coated with a dark grayish deposit which is readilyremoved by swabbing. Drying of, or pressure against this gray coatingshould be avoided inasmuch as its removal is thereby rendered morediillcult. The

purer grades of aluminum do not show this gray deposit but arepreferably swabbed to remove any loose precipitate that may survive the'rinsing operation. After the rinsing and swabbing, the

sheet aluminum is dried in any convenient manner and is then ready foruse without any further treatment.

While the plates of this invention may be made under widely variantconditions of bath, the optimum treatment, for consistent results, willobtain when the molal concentration of alkali hydroxide in the bath ismaintained a constant; that is, when the rate of the consumption ofalkali hydroxide is equal to the ratev of its liberation throughhydrolysis of the alkali aluminate. Although high molal concentrationsof alkali hydroxide cause rapid loss of weight, equilibrium of thebathat these concentrations requires a low ratio of surface area to-vo1ume;and di'- minishing returns result.

in the lower ranges of both the molal concentratin' of alkali hydroxideand the concentration of total alkali) .is inversely proportional to theequilibrium of molal concentration. Hence, it is in general preferred soto adjust the several variables as to maintain a relatively lowequiyield of treated surface per unit of bath volume in unit time isapproximated.

It w111 be obvious to those skilled in the art that the process of thisinvention is, as stated,

susceptible of wide variation and it is to be understood that theexamples of the preferred relationship of the several variables aregiven by way of illustration only.

The effect of the described treatment is threefold: it removes not onlythe superficial grease but also that grease which has been ground intometal in the process of rolling it into sheet form;

it so pits thesurface as to provide a ne grain; y and it leaves thesurface in an alkaline-reacting state. The complete absence of grease isof course of vital importance because any residual trace of grease wouldactas an ink-receptive image and would print a tone in the non-printedareas. Of vital importance also is the alkalinity of the resultantsurface, because upon this alkalinity depends that sensitivity to fattyacids which is necessary to the retention of direct images underprinting conditions and for long editions. The grain constituted by thepits, while not a prime essential, is nevertheless highly desirablebecause the dampening solution (with which the plate is wet betweensuccessive inking's during printing) is better retained by the platewhen the surface continuity of its plane surface is broken up by suchvgrain as is provided by the pits of this invention; and also,'althoughless importantly, because this grain provides a tooth which is ofadvantage when a direct image is delineated in pencil or crayon. 0n theother hand, the grain ofthe plates of this .invention is of so ne atexture that it is not objectionable when the image is formedphotographically from even the nest halftones.

While it is feasible to print 4from aluminum plates which have beentreated by the process of vthis invention merely tothe extent ofremoving the grease and of rendering the surface alkalinereacting butwithout any substantial pitting, such limited treatment is notrecommended because not all the advantages of this invention are fullyrealized when printing from plates wholly devoid of "grain. Theseadvantages are most fully realized when the-pitted area constitutesbetween 40 and 70% of the total surface. Under the conditions of theexamples above given, the following relationship obtains. The number ofthe pits per square millimeter of surface lies between two and vethousand. .The individual pits are generally spaced apart althoughoccasionally they encroach upon each other, and they are of generallyrounded and approximately circular contour. The mean diameter of thepits varies over the range lying between something less than 0.001 and0.015 millimeter; It is observed that in general the average of the meandiameters of the pits in a given area is roughly inversely proportionalto the concentration of pits in that area. Thus when the number of pitsper square millimeter is of the order of ve thousand, the average oftheir mean diameters is of the order of 0.008 milli- In fact, it hasbeen found that the productivity of a bath (atleast.

meter; and when there are but two thousand pits per square millimeter,the average of their mean diameters is of the order of 0.012 millimeter.The

ratio of pitted to non-pitted areas appears to vary much less than doesthe concentration of pits from one minute area to the next. The depth ofthe pits Vappears to be roughly proportional to their mean diameter. Thegrain constituted by the pits as described and within the above limitsis very fine when compared to the grain which is obtained by abrasiveattrition under gyrating marbles, as is the common practice; and 'l yyet it is fully e'ective to retain, against the squeegee action of theink rollers, a sufficiency of dampening solution. It is one of theadvantages of the plates of this invention that less dampening solutionis required to maintain the nonprinting portions clear from tone than isrequired by plates with the finest grain mechanically obtainable. Thisadvantageous behavior is thought to be due to the'high moistureretentivity of the open-textured or microporous material deposited inthe pits and sometimes over the entire surface.` The presence of thismicroporous ma.-

terial, in contrast to the extremely thin normal oxide, may be readilyobserved by microscopic inspection. If the microporous deposit islimited to the pitted areas of the plate, its presence though not itsporosity may be observed by inspection of a microtomed section undervertical incident illumination. If the deposit extends beyond the pittedareas, it may be discerned without sectioning, by microscopic inspectionundervertical in`- cident illumination. In, either case, whether limitedto the .pitted areas or extended over the entire surface of the plate,the relative thickness of the deposit may be estimated microscopicallyunder incident dark field illumination.

The microporosity of the deposit may be demonstrated in terms of itshigh adsorptive capacity for finely divided pigment, as carbon black forinstance, by the following procedure, which also serves to show thecontrasting behavior of the normal oxide coating on the non-pitted areaswhen the latter are free from the microporous deposit of the process ofthis invention. Usual black lithographie ink is first well rolled onto arestricted area of the plate and then washed out ride for instance. Theso-treated area, when microscopically viewed` at a magnification ofabout 100 diameters, will be observed to be of undiminished metallicbrilliancy in the nonpitted areas and dark gray in the pitted areas,Repeated washings with solvent are not e'ective to appreciably lightenthe grayness of the pitted areas; from which observation'it is concludedthat the pigment of the ink'is irmly adsorbed within micropores of thedeposit on the pits.

When the microporous deposit covers the nonpitted as well as the pittedareas, the abovedescribed procedure aifords a test for the continuity ofthe deposit as well as for` its microporosity. If the deposit iscontinuous over the entire surface, the demarcation of the inked area,after washing the ink therefrom, will still be macroscopicallydiscernible; although contrast .-ing shade. In Figure 4 of the drawing,the

darker areasof the coating, corresponding to the pits I5, are indicatedby the numeral I'I andl the lighter areas by the numeral I8. I'heapparent density of the coloring, residual after the above-describedinking and solvent treatment, is believed to be proportional to thethickness of the coating, whichis greatest in the isolated areasoverlying the deeper pits and is least in the expanses of the unpittedor very slightly pitted areas lying therebetween.

'I'he microporous deposits on thev plates of this invention may bedistinguished from both normal atmospheric oxide and from anodicaluminum oxide by two distinct characteristics. Thus, when scratchedwith a needlepoint, the said deposit appears under the microscope andata magnification of 100 diameters or so, to consist of whitishparticles suggestive of a relatively delicate and iinely crystalline-material and distinctly not comparable to the powdery product of asimilarly scratched hard amorphous mass, such as the relatively densealuminum oxide produced by anodic deposition. It is thought that theexpression "microporous crystalline oxide is -useful in distinguishingthe oxide deposited by the process of this invention both from normalatmospheric oxide and from oxide produced anodically.

The second distinguishing characteristic is vthat the microporouscrystalline oxide deposited fatty acids of direct image-formingmaterialsv may react; vand thus is avoided the necessity for sensitizingor counter-etching by the user. Another advantage is manifested by thesubstantially greater life under printing conditions of an image deriveddirectly from a greasy imageforming substance containing a free fattyacid reactive with said alkali over the life of any direct imageobtainable in the absence of said alkaline component in the deposit.

If a restricted portion ofa plate of this invention be treated withacid, as dilute phosphoric acid for instance; and it be then thoroughly,washed and dried, and an image thereafter directly delineated upon boththe normal surface and the acid-'treated surface .under likeVconditions, it will be found that the printing life of the image on thenormal surface greatly exceeds that of the corresponding image on theacid treated surface. That the relative capacities for which issensitized by the process of this invention and of a portion of thatsurface as modiflecl by desensitization with acid indicate not merelya'difference vin degree in sensitivity but a diiference in kind 'ofsurface may be demonstrated by a simple and expeditious bench test.Thus, if a plate of this invention be desensitized over part of its areawith an acid, as phosphoric for instance; and if, after thorough Washingand drying, it be rolled up with a usual lithographic ink and thenflooded with a weakly acidic etch, comprising .an acid phosphate such asthat disclosed in U. S. Patent No. 2,003,268,

'direct-image retention on the surface of a plate I may require gentleswabbing for their complete removal. The ink on the sensitized or normalsurface of the plate will be found to be undisturbed by the acidic etch.It is believed that the retention of the ink by the surface of theplates of this invention, against the action of the acidic etch, is dueto chemical reaction between the fatty acids of the ink and the alkalicomponent of the oxide lm coating the surface of these plates.

The presence of alkali, as a component of the oxide deposited on thesurface of these plates, may be demonstrated colorimetrically; but,because of the extremely small amount of deposited oxide, thisdetermination is tedious and difficult. A characteristic differencebetween the normal alkaline surface of the plates of this invention andsuch surface after desensitization by an acid etch may otherwise bedemonstrated by observing the relative wettability of these surfaces byaqueous ethyl alcohol comprising to 55% of water. A drop of this mixturewill spread immediately when applied to the normalA surface of theplate, but will not spread when applied to the desensitized oracid-treated/ surface.

While the planographic printing plates of this invention which havetheir entire surfaces covered with a. continuous layer of microporousdeposit (see Fig. 3) are particularlyuseful for the reception andretention of images formed from the less reactive grades of carbon paperand for free hand work with the drier grades of crayon,

lthey are not particularly satisfactory for photographic reproduction.Because of the very opentextured surfacing, which advantageously retainsdampening solution, the unexposed photo-sensitive material with whichthey are sensitized for photographic reproduction cannot be whollywashed out of the porous surface layer on development; and hence theobtainment fromvsuch plates of clear tone-free whites in thenon-printlng areas is generally diflicult and more particularly so withfine halftones. When an image or part of an image is to be formedphotographically on plates of this invention, itis preferable to useplates on the surfaces of which the microporous deposit is segregatedwithin the pits and generally below the level of the non-pitted areas(see Fig. 2) It is believed that even though some of the unexposedphotosensitive material may remain in the micropores of the depositwithin the pits, the dampening solution is so firmly retained in thepits against the squeegee action of the ink rolls as to eifectivelyprotect from ink contamination such traces of photosensitive material asmay have been there retained.

I claim:

1. In aprocess of preparing planographic printing plates from sheetaluminum, that step which comprises reacting saidaluminum with an alkalihydroxide in hot solution with an alkali aluminate, whereby the surfaceof said aluminum is freed from grease and is sensitized to fatty acids.f 2. 'I'he process of preparing planographic printing plates from sheetaluminum which comprises immersing said sheet aluminum in a solution-of`an alkali aluminate until the surface of said aluminum is freed fromgrease, is minutely pitted. and is sensitized to fatty acids.

3. 'I'he process of preparing vphanographic printing plates from sheetaluminum which comprises immersing said sheet aluminum in a solution ofan alkali aluminate until the surface of said aluminum is` freed fromgrease, and is grained with minute pits coated with a micropo'ousaluminum oxide comprising a trace of al ali.

4. The process of preparing planographic printing plates from sheetaluminum which comprises immersing said sheet aluminum in a solution ofan alkali aluminate until the surface of said aluminum is freed fromgrease, is grained with minute pits, and is coated with'a microporousaluminum oxide comprising a trace of a1- kali.

5. A planographic printing plate of sheet aluminum characterized byminute pits of the order of from two to ilve thousand per squaremillimeter and of generally .rounded contour in an otherwisesubstantially plane surface, the vprinting surface of said plate beingalkaline reacting and highly sensitive to fatty acids.

6. A planographic printing plate of sheet aluminum characterized byminute pits of the order of from two to five thousand per squaremillimeter and of generally rounded contour in vvan otherwisesubstantially plane surface, said pits being partially filled with adeposit of microporous oxide and said surface being alkaline reactingand highly sensitive to fatty acids.

7. A planographic printing plate of sheet aluminum characterized byminute pits of the o rder of from two to ilve thousand per squaremillimeter and of generally rounded' contour inI an otherwisesubstantially plane surface, said pits and surface'being coated with adeposit of crystalline -microporous oxide thicker in the pits than overthe substantially plane surface comprising a trace of alkali and beinghighly sensitive to fatty acids.

8. A planographic printing plate of sheet aluminum characterized bybetween two and ve thousand minute pits of generally rounded contour tothe square millimeter in an otherwise substantially plane surface, saidpits having a mean diameter o f the order of five one-thousandths of vamillimeter and said surface being alkaline reacting and highly sensitiveto fatty acids.

9. A planographicprinting plate of sheet aluminum characterized bybetween two and ilve thousand minute pits of generally rounded contourto the square millimeter in anotherwise'substantially planesurface,`said pits having a mean diameter of the order of ilveone-thousandths of a millimeter and said pits and surface being coatedwith a deposit of a crystalline microporous oxide comprising a trace ofalkali and being highly sensitive to fatty acids, said deposit having amean thickness of between one and three onethousandths of a millimeter.

WILLIAM B. WESCOTI.

